The present disclosure relates to capillary tubing heat exchangers. More particularly, it relates to capillary tube bundles useful in extracorporeal blood circuit heat exchangers, and related methods of manufacture.
Fluid-to-fluid heat exchangers are used in many different industries, and are typically constructed in conjunction with the intended end use. For example, a heat exchanger is an important component of an extracorporeal or cardiopulmonary bypass circuit. As a point of reference, an extracorporeal blood circuit is commonly used during cardiopulmonary bypass (i.e., a heart-lung bypass machine) to withdraw blood from the venous portion of the patient's circulation system and return the blood to the arterial portion. The extracorporeal blood circuit generally includes a venous line, a venous blood line reservoir, a blood pump, an oxygenator, a heat exchanger, an arterial line, and blood transporting tubing, ports, and connection pieces interconnecting the components. The heat exchanger regulates a temperature of the extracorporeal blood as desired. For example, the heat exchanger can be located upstream of the oxygenator and operated to cool the blood arriving from the patient prior to oxygenation; alternatively, the heat exchanger can be operated to warm the extracorporeal blood.
Regardless of the direction of heat transfer between the heat exchanger and the patient's blood, extracorporeal blood circuit heat exchangers generally consist of metal bellows and a multiplicity of metal or plastic tubes (capillary tubes); a suitable heat exchange fluid, such as water, is pumped through the tube lumens while the blood flows about the tube exteriors. The heat exchange fluid can be heated or cooled (relative to a temperature of the blood). As blood contacts the tubes, heat transfer occurs between the blood and the heat exchange fluid in an intended direction. Alternatively, blood flow can be through the tube lumens, with the heat exchange fluid flowing about the tube exteriors.
So as to have minimal impact on the circuit's prime volume, the extracorporeal heat exchanger is desirably as small as possible, while still providing high heat exchange efficiency. To meet these requirements, the capillary tubes are micro-diameter or fiber-like (e.g., outer diameter no greater than about 0.05 inch). The heat exchange fluid is fluidly isolated from blood of the extracorporeal circuit by a wall thickness of the capillaries, keeping the fluids separate but allowing the transfer of heat from one fluid to the other.
A common capillary tubing format pre-assembles a large number of the micro-diameter tubes into a mat. The capillary tubes are knitted, woven or otherwise held together with threads or stitching forming the warp of the mat. For heat exchanger applications, the capillary tube mat must be bundled together in some fashion to form a capillary tube bundle. Typically, the mat is wrapped or rolled around a core or mandrel. As the mat is continuously wound about the mandrel, the mat wraps or winds onto itself, resulting in a series of radially increasing layers. The capillary tubes of the mat are conventionally “biased” so that the tubes are not parallel with a width of the mat. Two layers of the mat with opposite bias angles can be simultaneously wound on the core to prevent the capillaries of subsequent layers from nesting in the gaps between capillary tubes of a preceding layer as the mat is wrapped onto itself.
While highly viable, capillary mat-based heat exchangers have certain drawbacks. For example, capillary tubing mats are expensive due to the complexities of the knitting or weaving process. Further, the size, bias, materials, spacing, etc., of the capillary tubes is fixed, such that possible benefits available with varying one or more of these parameters is unavailable.
Capillary tube bundles are also used in other mass transfer devices, and in particular blood oxygenators. As a point of reference, the capillary tubing employed with oxygenators is markedly different from that of heat exchangers; oxygenator capillary tubing is porous or semi-permeable, whereas heat exchanger capillary tubing is fluid impermeable. These differences affect fluid flow properties and may impact manufacturing techniques. In any event, a woven capillary tube mat akin to the above descriptions can be used to form an oxygenator capillary tube bundle. In an alternative approach, a single capillary tube, or ribbon of capillary tubes, is directly wound onto a rotating core, generating a helical wind pattern. One such oxygenator bundle winding technique is set forth in U.S. Pat. No. 4,975,247 that otherwise describes the means by which to wind an oxygenator capillary tube with specialized winding equipment. Regardless of whether such techniques are applicable to heat exchanger capillary tube bundles, the helical winding format of the '247 patent (and other similar techniques) results in an interleaving of the capillary tubes within each layer. In many instances, this interleaving may be less than optimal for extracorporeal blood circuit heat exchanger functioning and performance.
In light of the above, a need exists for improved heat exchanger capillary tube bundle manufacturing techniques that combine low cost and direct control over production, as well as for the capillary tube bundles and heat exchangers resulting from such techniques.